In the related art, physical vapor deposition, such as arc ion plating and sputtering, is widely known as a technique for coating a surface of a substrate, such as a mechanical component, a cutting tool, or a slidable component, with a thin film for the purpose of improving abrasion resistance, sliding properties, and protective functions. For arc ion plating, cathode-discharge-type arc evaporation sources are used.
A cathode-discharge-type arc evaporation source generates an arc discharge on a surface of a target, which is a cathode, so as to instantaneously melt the material constituting the target. Then, the ionized material is drawn toward a surface of a substrate, which is an object to be processed, so as to form a thin film on the substrate. Since such an arc evaporation source has characteristics in which the evaporation rate of the target is high and the ionization rate of the evaporated material constituting the target is high, a dense film can be formed by applying bias to the substrate during the deposition process. Therefore, the arc evaporation source is industrially used for forming an abrasion-resistant film for a cutting tool or the like.
However, in the case where an arc discharge is to be generated between the cathode (target) and an anode, when evaporation of the target centered on an electron emission point (arc spot) at the cathode side occurs, the target melts and is released from near the spot, and the molten material adheres to the object to the processed, sometimes resulting in a reduced degree of surface roughness.
The amount of the molten target material (macro-particles: electrically neutral droplets) released from the arc spot in this manner tends to be suppressed when the arc spot moves at a high rate, and this moving rate is known to be affected by a magnetic field applied to the target.
Furthermore, since target atoms evaporated due to the arc discharge are ionized within an arc plasma, there is a problem in that an ion trajectory extending from the target toward the substrate is affected by the magnetic field between the target and the substrate.
In order to solve these problems, there have been proposed the following attempts to control the movement of the arc spot by applying a magnetic field to the target. For example, PTL 1 discloses a technique in which a ring-shaped magnetic generating mechanism (permanent magnet, electromagnetic coil) is disposed around the target so as to apply a vertical magnetic field to the surface of the target. PTL 2 discloses a technique in which a mechanism (electromagnetic coil) for generating a magnetic force for converging the ionized material constituting the target is disposed in front of the target so that the ionized material is efficiently converged in the direction toward the substrate. PTL 3 discloses a technique in which a permanent magnet is set in the center of the rear face of the target in the arc evaporation source, a ring-shaped magnet having a different polarity is disposed at the rear side of the target so as to surround the permanent magnet, and an electromagnetic coil substantially having the same diameter as the ring-shaped magnet and forming components of a magnetic field that keeps an arc discharge confined is provided. PTL 4 discloses a technique in which a magnetic field that is parallel with the surface of the target is generated by a rear-surface electromagnetic coil and a ring-shaped magnet disposed around the target.
However, in the magnetic generating mechanism in PTL 1, since magnetic lines of force from the surface of the target extend toward a ring-shaped magnet, many of the ions are induced toward the magnet. In addition, since magnetic lines of force extending toward the substrate in front of the target significantly diverge from the direction toward the substrate, the evaporated and ionized material constituting the target cannot efficiently reach the substrate.
In the technique discussed in PTL 2, although magnetic lines of force extend toward the substrate, since it is necessary to dispose the electromagnetic coil, which is large is size, between the target and the substrate, the distance between the target and the substrate inevitably increases, resulting in a reduced deposition rate.
Furthermore, although an arc discharge tends to occur by priority at a point where perpendicular components of a magnetic field become zero (i.e., components of a magnetic field that are perpendicular to the surface of the target), since the point where the perpendicular components of the magnetic field become zero is trapped at a substantially intermediate region between the permanent magnet and the ring-shaped magnet in the arrangement disclosed in PTL 3, it is difficult to control the arc discharge to an inner peripheral region relative to the aforementioned point even by using the electromagnetic coil, and the utilization efficiency of the target is not high. Moreover, with the arrangement in PTL 3, since there are no magnetic lines of force extending forward from the target, the ions emitted from the target cannot be efficiently converged in the direction toward the substrate.
PTL 4 only discloses an embodiment in which the inner diameter of the electromagnetic coil is smaller than the diameter of the target. In this embodiment, since magnetic lines of force tend to disperse outward from the target, it is conceivable that ions cannot be converged efficiently. Furthermore, in order to move an arc plasma discharge at a high rate, it is necessary to increase the strength of the magnetic field that is parallel with the surface of the target. To achieve this, the electromagnetic coil (or a magnetic yoke) needs to be increased in size, and a large electric current needs to be supplied to the electromagnetic coil. Since this leads to an increase in size of the evaporation source, this is not industrially desirable.
FIG. 5 is a distribution diagram of magnetic lines of force in the technique discussed in PTL 4 (i.e., a technique in which an electromagnetic coil having an inner diameter smaller than the diameter of the target is disposed at the rear side of the target and a core is disposed within the inner periphery of the electromagnetic coil, which will simply be referred to as “comparative technique” hereinafter).